A process for preparing organohalosilanes from metallic silicon and halogenated hydrocarbons using a copper or copper compound catalyst was first disclosed in U.S. Pat. No. 2,380,995, and is today called the Rochow reaction after the name of the inventor. The silicone industry has carried out organohalosilane synthesis using this direct synthesis process ever since it was invented. That is, organochlorosilanes such as methylchlorosilane are synthesized by the Rochow reaction in which an organic halide such as an alkyl halide (e.g., methyl chloride) or a halogenated aryl compound (e.g., halobenzene) is passed through metallic silicon and a catalyst component composed of a copper catalyst and a small amount of co-catalyst to induce a direct reaction in a vapor phase. In this reaction, because the cost of metallic silicon accounts for a large portion of the raw material costs, it is essential to increase the conversion of metallic silicon and also to maintain reaction conditions in such a way as to bring the formation ratio of the many by-products that generally form together with the main product into line with the demand-supply balance for organochlorosilanes. Industrially, the reaction is generally carried out in a reactor such as a fluidized bed reactor, vibrating fluidized bed reactor or stirred tank reactor while adding the catalyst component. Although activation to bring the reaction to a steady state takes a long time, the steady state is relatively shortlived. Accordingly, it is important to minimize the decline in activity (i.e., the rate of decline in the reaction rate and selectivity) due to the accumulation of deactivated catalyst component as the reaction proceeds in order to enable long-term operation, and thereby increase the conversion of metallic silicon to useful silanes.
The aluminum present as an impurity in industrial metallic silicon reportedly has a large impact on the reaction rate and selectivity. For example, according to Norwegian Patent No. 169831, the ternary phase FeAl3Si2 in silicon provides enhanced reactivity, and the quaternary phase Fe4Si6Al4Ca provides enhanced selectivity. However, the reactivity and selectivity cannot both be increased. That is, it is well known that in the Rochow reaction the aluminum present as an impurity in the industrial metallic silicon serving as one of the starting materials is essential for increasing the catalytic activity, yet it lowers the selectivity for diorganodihalosilanes that are in high demand. The form and reactivity of aluminum present in metallic silicon has been the subject of considerable research and debate.
British Patent No. 2153697 teaches that both the reactivity and selectivity of a direct synthesis process are increased by the use of a copper catalyst comprising a mixture of copper, Cu2O and CuO, from 200 to 5,000 ppm of a tin-containing compound, and from 50 to 5,000 ppm of aluminum or an aluminum-containing compound. Unfortunately, this approach fails to provide significant increases in reactivity and selectivity.
H.M. Rong has proposed, in Norwegian Patent No. 950760, a process for the production of alkylhalosilanes by reacting elemental silicon with an alkyl halide at an elevated temperature in the presence of a copper-based catalyst and an optional promotor. However, such a process does not achieve significant increases in reactivity and selectivity. Although the examples of aluminum cited in this prior art include metallic aluminum, aluminum alloys, aluminum-containing silicon alloys and solid aluminum-containing compounds, none of these has sufficient activity by itself. Various active forms of these aluminum substances have been proposed, but no accepted view yet exists on their efficacy, nor have any specific procedures been described for activating such aluminum substances.
Thus, although aluminum has been the subject of considerable research, such efforts have involved merely the use of aluminum impurities in metallic silicon by default as the reaction promotor. No art disclosed to date has proposed activating the aluminum in metallic silicon, which is present primarily as aluminum silicide, to put it to effective use. Because the prior art uses as a starting material aluminum-containing metallic silicon, which has both advantages and drawbacks in the Rochow reaction, the concentration of aluminum within the reaction system is difficult to control, as is also the starting material itself.
It is therefore an object of the invention to provide a process for preparing organohalosilanes by the Rochow reaction that can shorten the time required for activation, increase the selectivity for desired diorganodihalosilanes, prolong the steady state of the reaction and improve conversion of the silicon.
The inventor has found it to be effective, when preparing organohalosilanes by the Rochow process, to add to the reaction system at least one promotor selected from among activated aluminum, activated aluminum alloys and activated aluminum carbide.
As noted above, there is little doubt that at least some of the aluminum present as aluminum alloy within metallic silicon reacts with halogenated hydrocarbons to form aluminum halides. The aluminum halides reportedly react with the oxide film present on the surface of the metallic silicon, inducing a surface-activating effect. Moreover, it has also been reported that the presence of aluminum halides increases the vapor pressure of copper halides that form from the copper catalyst, thus facilitating diffusion of the copper catalyst and ultimately promoting the catalytic effect of the copper. In any case, it appears to be indisputable that the presence of aluminum promotes the reaction.
Yet, at the same time, the aluminum halides that form as by-products are very strong Lewis acids, and are indeed familiar as catalysts for organohalosilane disproportionation reactions. Hence, the presence of excess aluminum halide gives rise to disproportionation within the reaction system of the diorganodihalosilanes which are primary constituents of the reaction product and are preferably obtained in the highest possible yield, resulting in an undesirable increase in silane by-products such as monoorganotrihalosilanes and triorganomonohalosilanes.
More specifically, in the activating reaction stage at the beginning of the Rochow reaction, the formation of much aluminum halide is advantageous because it is necessary to activate the catalyst component, but once a steady state has been reached, the formation of little aluminum halide is preferred. Achieving in this way the mutually conflicting goals of improved reactivity and improved selectivity requires very close and careful control of the reaction, yet the prior art carries out reactions which attempt to achieve this delicate effect using the aluminum impurities already present within the metallic silicon, such as alloys with silicon. Such aluminum impurities within metallic silicon originate from impurities within the silica starting materials used in the metallurgical production process, and so are not uniformly present throughout the metallic silicon. Rather, they form intermetallic compounds with silicon and other metals or compounds with nonmetallic elements, and are dispersed throughout the silicon as impurity zones. The slow rate of aluminum halide formation from reactions with organic halides has made it necessary to take one of two approaches: either use a higher aluminum content than would otherwise be warranted or use metallic silicon having a low aluminum content to achieve good selectivity in an activation period having a long initial stage. Hence, it is impossible in practice to control the active aluminum within the catalyst component to the required level, and such control by itself cannot increase both the reactivity and the selectivity.
Upon investigating instead reactions between what is referred to in the prior art as xe2x80x9cactive aluminumxe2x80x9d and halogenated hydrocarbons, the inventor learned that within a temperature range of 250 to 400xc2x0 C. such reactions either do not proceed quantitatively (that is, to a sufficient degree) or do not proceed at all. However, when at least one aluminum substance selected from among aluminum, aluminum alloys and aluminum carbide is premixed with the copper compound, the inventor found that the aluminum can be made to react quantitatively with halogenated hydrocarbons within this temperature range to form aluminum halides. This showed that activated aluminum, activated aluminum alloy and activated aluminum carbide have an effective action on the reactivity and selectivity of the reaction. That is, the inventor discovered that aluminum, aluminum alloy and aluminum carbide react quickly with halogenated hydrocarbons in the presence of a copper compound to form aluminum halides and hydrocarbons. Moreover, because the reaction proceeds quantitatively, sufficient activation can be achieved with addition of the minimum required amount of the aluminum substance. The inventor also found that, in the presence of a catalyst composed solely of a copper compound, aluminum carbide and aluminum alloys such as aluminum silicide, which have a much lower reactivity with halogenated hydrocarbons than does metallic aluminum, are activated by the addition of a small amount of co-catalyst (e.g., metallic tin, zinc, antimony, phosphorus, iron) with respect to the copper compound and thus rapidly react to form an aluminum halide. In this reaction as well, because the aluminum present in the form of an alloy or the aluminum carbide reacts quantitatively, sufficient activation of the Rochow reaction can be achieved by adding the aluminum substance in the minimum required amount. Another unanticipated discovery the inventor made was that the activated aluminum, activated aluminum alloy and activated aluminum carbide react more rapidly with methyl halides to form aluminum halides and hydrocarbons, thereby reducing the amount of hydrogen that generally forms as a by-product when conventional aluminum compounds are used. This has the effect of suppressing the formation of organohydrogenhalosilanes (Si-H group-bearing silanes) that are by-products of direct synthesis, thereby resulting in an increased selectivity.
Thus, although the prior art makes no mention of the deliberate use of aluminum as a co-catalyst, the inventor has found the use of quick-acting aluminum to be effective and important in controlling the reactivity of the catalyst component which changes from moment to moment.
Accordingly, the invention provides a process for preparing organohalosilanes comprising the step of reacting metallic silicon with a halogenated hydrocarbon at a temperature of 250 to 400xc2x0 C. in a stirred tank reactor or a fluidized bed reactor and in the presence of a copper or copper compound catalyst and at least one promotor selected from among activated aluminum, activated aluminum alloy and activated aluminum carbide.
Because the aluminum promotor is independent of impurities in the metallic silicon starting material and quick-acting and because it does not remain or accumulate within the reaction system, the inventive process is able to achieve a reaction having a high selectivity and a high conversion. This in turn makes it possible also to facilitate control of the starting materials.